Are the valve and pipework non magnetic?How reliable does it need to be? If the reading is wrong will the reactor blow up, or is it just an annoyance?
A sturdy way to detect the lever, even in a humid environment and which is not in the way at any moment ? That might not be possible. Your insurance might be void if you try to add something.
But I suspect that a microswitch will make it difficult to use the valve.
I'll consider the magnet reed relay solution. Got reed relays but no small strong magnets.
Not if it is properly designed, like these commercially available ball valves with limit switches.